Is it possible for you to buy them some gifts and give it to them on their birthday or Xmas day? You could try doing it in secret and tell the kids not let their father see them until a short time after the occassion.
Our Grandmother would send us kids a little package a month before or after our brithday each year, it used to infuriate our dad and she would use the excuse that she felt she needed to recognize each of us individually at least once a year. Then at Xmas time we would holiday with her and at the end of it she would have a special dinner and give us all presents. It was so thoughtful and puts a smile on my face when I think about it now.